FIG. 1 illustrates a library system employing an X number of intermediate control units (“ICU”) 40 (e.g., sixteen ICUs 40) with each ICU 40 being physically connected to a Y number of library devices 21 of a medium changer library 20 (e.g., sixteen library devices). A host library controller as known in the art (not shown) includes an X number of ports with each port being physically connected to an associated ICU 40 whereby the known host library controller systematically or randomly selects one of the ports for communicating an issued command from a medium changer command set to medium changer library 20 via its associated ICU 40 and library devices 21. For example, the known host library control could systematically or randomly select a port for communicatingi an issued command from a medium changer command set to medium changer library 20 via ICU 40(1) and one of the Y library devices 21(1).
In operation, the selected port communicates the issued medium changer command to its associated ICU 40. Upon receiving the recently issued medium changer command, the ICU 40 will queue the recently issued medium changer command for communication to medium changer library 20 depending on a priority status of the medium changer command. For example, medium changer non-move commands will typically have a higher priority than medium changer move commands and therefore will be provided with a higher priority status in the queue. To manage the queue, the ICU 40 employs a specific number of threads (e.g., three threads) for communicating all issued medium changer commands stored in the queue to medium changer library 20. As such, if the number of previously issued medium changer commands stored in the queue exceeds the specific number of threads, then the recently issued medium changer command will be stored in the queue depending on its priority status until one of the threads is available to send the recently issued medium changer command to medium changer library 20.
It is not desirable for a host library controller to wait an infinite amount of time after issuing a medium changer command for a response from an ICU 40 indicating a successful communication of the medium changer command to medium changer library 20. In view of the fact that a host library controller can not predict how long it may take an ICU 40 to complete a communication of an issued medium changer command to medium changer library 20 and in view of the fact that a host library controller will not know the operational status of medium changer library 20 unless a precise error has been reported, a host library controller as known in the art will more than likely prematurely time out the medium changer command that results in unnecessary execution of a error recovery procedure by the known host library controller.